This invention relates to a recording sheet conveying device, for instance, in a laser printer or electrophotographic copying machine which is adapted to convey a recording sheet while holding it in place, and more particularly to a recording sheet conveying device which is adapted to convey a recording sheet while retaining it in place using an electrostatic attraction force.
For instance in a color copying machine, monochromatic toner images of yellow, cyan and magenta formed on its photo-sensitive drum are transferred on a recording sheet one after another, to form a full-color image on it. For this purpose, it is necessary for the color copying machine to have a recording sheet conveying device to send one and the same recording sheet to a toner image transferring section repeatedly. A conventional recording sheet conveying device of this type is such that an electrostatic attraction force is used to retain a recording sheet on a dielectric sheet which is wound on a rotary drum.
An example of the conventional recording sheet conveying device is as outlined in FIG. 18. In FIG. 18, reference character a designates a dielectric sheet on which a recording sheet b is retained so as to be conveyed; c, a charger arranged on the side of the inner surface of the dielectric sheet a and connected to a DC power source d; and e, an electrically conductive deflector roll which is confronted through the dielectric sheet with the charger to push the recording sheet against the outer surface of the dielectric sheet.
The recording sheet b is retained on the dielectric sheet a as follows: When the recording sheet b passes through the gap between the dielectric sheet a and the deflector roll e, the charger is activated to charge the inner surface of the dielectric sheet a (positive charges being injected thereinto in the case of FIG. 18), as a result of which charges opposite in polarity those on the inner surface (negative charges in the case of FIG. 18) flow into the outer surface of the dielectric sheet through the deflector roll. Hence, electrostatic attraction forces are induced between the dielectric sheet and the recording sheet b, so that the latter b is electrostatically attracted to and retained on the dielectric sheet a.
Heretofore, while the recording sheet b is passing through the gap between the dielectric sheet a and the deflector roll e, the charger c is kept activated so that the electrostatic attraction forces act on the whole area of the recording sheet b.
When the electrostatic attraction forces act on the whole area of the recording sheet as was described above, the recording sheet is liable to be wrinkled. This phenomenon is significant particularly in the case where the recording sheet deformed by moisture is to be retained on the dielectric sheet.
This difficulty is serious with a recording sheet conveying device which is used to convey a recording sheet to the toner image transferring section. That is, if a recording sheet sent to the transferring section has been wrinkled, part of the toner image may not be transferred from the photo-sensitive drum onto the recording sheet because of the wrinkles. Accordingly, the resultant image is unsatisfactory in picture quality, having blank parts. Thus, the copying machine or printer with the recording sheet conveying device would be unreliable in operation.